The present invention relates to an output current driver circuit which is provided in a bipolar integrated circuit serving as an interface driver or the like, and, more particularly, to a circuit wherein an output transistor circuit comprises two darlington-connected NPN transistors.
Known conventional circuits for increasing the output current of an output current driver circuit of a bipolar integrated circuit can be classified into two types. The first type is an output transistor circuit comprising two NPN transistors Q.sub.1 and Q.sub.2 darlington-connected as is shown in FIG. 1. The second type is an output transistor circuit comprising three or more NPN transistors Q.sub.1, Q.sub.2, . . . , Q.sub.n darlington-connected as is shown in FIG. 2.
In FIG. 1, when an input current to an input terminal 1 assumes I.sub.in, and current-amplification factors of transistors Q.sub.1 and Q.sub.2 assume h.sub.FEQ1 and h.sub.FEQ2, an output current I.sub.out of an output terminal 2 will be EQU I.sub.out =I.sub.in .times.h.sub.FEQ1 .times.h.sub.FEQ2 ( 1)
As understood from equation (1), according to the transistor circuit of FIG. 1, it is possible to obtain an output current which is h.sub.FEQ2 times larger than the output current (I.sub.in .times.h.sub.FEQ1) of the circuit comprising only output transistor Q.sub.1.
Assuming that the threshold voltages of transistors Q.sub.1 and Q.sub.2 are V.sub.BEQ1 and V.sub.BEQ2 and that a saturation voltage across the collector and emitter of transistor Q.sub.1 is V.sub.CE(SAT)Q1, the threshold voltage V.sub.THt and the output saturation voltage V.sub.CE(SAT)t of the output transistor circuit will be EQU V.sub.THt =V.sub.BEQ1 +V.sub.BEQ2 ( 2) EQU V.sub.CE(SAT)t =V.sub.CE(SAT)Q1 +V.sub.BEQ2 ( 3)
On the other hand, in the circuit of FIG. 2, since there are a number of darlington-connected output transistors, output current I.sub.out, larger than that of the circuit of FIG. 1, is derived.
However, in the circuit of FIG. 2, assuming that the threshold voltages of transistors Q.sub.1, Q.sub.2, . . . , Q.sub.n are represented by V.sub.BEQ1, V.sub.BEQ2, . . . , and V.sub.BEQn and that a saturation voltage across the collector and emitter of transistor Q.sub.1 is represented by V.sub.CE(SAT)Q1, threshold voltage V.sub.THt and output saturation voltage V.sub.CE(SAT)t of the output transistor circuit will be EQU V.sub.THt =V.sub.BEQ1 +V.sub.BEQ2 + . . . +V.sub.BEQn ( 4) EQU V.sub.CE(SAT)t =V.sub.CE(SAT)Q1 +V.sub.BEQ2 + . . . +V.sub.BEQn ( 5)
Therefore, the values of V.sub.THt and V.sub.CE(SAT)t increase by amounts equal to the number of darlington-connected transistors. Therefore, the use of the circuit is limited. Namely, when threshold voltage V.sub.THt is large, the operating power source voltage of the circuit must be increased. When output saturation voltage V.sub.CE(SAT)t is large, the electric power consumption of the circuit increases, giving rise to such problems as excessive heat generation and the like. Thus, the increase in number of transistors limits the use of the circuit.
Therefore, in order to increase output current I.sub.out in the circuit arrangement of FIG. 1 in which only two transistors are used, conductivities of output transistors Q.sub.1 and Q.sub.2 may be ehhanced (in particular, the size of transistor Q.sub.2 is enlarged). However, this method results in an undesirable increase in the size of transistors Q.sub.1 and Q.sub.2, when an integrated circuit is formed, so that the circuit chip must also be enlarged. Consequently, the cost of the resulting chip is higher. Therefore, this method is unsuitable.